basically using YAML instead of database is a situation where you don't need a dynamic data such as I had a task of customizable client interface such as colors of links, visited links, main background, header background color, header text color, etc. Also we have some default colors defined and also need to provide option to client to reset it to website default colors. Doing so storing a static data in database makes not much sense (it works though but not a good option where you have already 50+ models and need to handle such a situation more than once), so I made a YAML file have default colors. Hopes this help you.

Yes, it's especially good for simple configuration associated with the web application. Here is a great Rails screencast for creating and using a YAML config file: http://railscasts.com/episodes/85-yaml-configuration-file

In addition, if you need to store configuration-like data with your database records, you can use the useful Rails method serialize: http://api.rubyonrails.org/classes/ActiveRecord/Base.html#M002229